Conventionally known techniques for preventing tap water from being supplied as "red water" include caustic soda injection, soda ash injection, and slaked lime injection. Of these, slaked lime injection is recently attracting attention and is increasingly adopted in water purification plants of tap water suppliers, because not only does slaked lime serve to increase the pH of water to be treated but also it is said that calcium contained in slaked lime as a major component is good for human health, and because a technique of injecting slaked lime not as a powder but as an aqueous solution has been developed.
With respect to the continuous production of an aqueous solution of slaked lime, a method using such an apparatus is disclosed in, for example, U.S. Pat. No. 4,389,376. The method described in U.S. Pat. No. 4,389,376 uses an apparatus comprising a tank in which its upper part is cylindrical or rectangular while its lower part is in the shape of an inverted truncated cone, a water supply piping and an insoluble material discharge piping both connected to a bottom part of the tank, agitator blades revolving horizontally and disposed over the outlet end of the water supply piping, and an overflow solution-receiving part provided by the edge of the tank top and having a discharge piping for taking out aqueous slaked lime solution. In this method, a layer of accumulated slaked lime particles is formed in the tank and, with low-speed stirring, water is supplied at a constant rate from the lower part of the tank and an aqueous solution of slaked lime is taken out from the upper part of the tank.
However, in such a slaked lime-dissolving tank in which only an agitator has been disposed, water feeding should be conducted at such a low rate that the water in the upper part of the tank ascends at a rate of about 0.2 mm/sec at the most because there is the possibility that too high a water feeding rate may cause the effluent aqueous solution of slaked lime to carry part of the slaked lime slurry blanket. Such a water ascent rate is too low from the standpoint of practically using the apparatus in large-scale water treatment plants. Therefore, the above conventional apparatus has a drawback that for use in large-scale water treatment plants, the apparatus should have an increased size or should be installed in a large number, resulting in the necessity of a large installation area and in an increased construction cost.